1. Field of the Invention
The present invention relates to a semiconductor device used in the Extremely High Frequency (EHF) band, and more particularly to an electrode arrangement in a switching element used in the EHF band, such as a Schottky barrier diode.
2. Related Art
There has been known a GaAs Schottky barrier diode used as a switching element of a Monolithic Microwave Integrated Circuit (MMIC) available for various systems such as communication systems or radar systems. The Schottky barrier diode has a valuable ability for providing a switching element having a high isolation characteristic by taking advantage of its smaller on-resistance than that between the drain and source of an FET.
FIG. 13A shows an electrode arrangement in a conventional Schottky barrier diode provided on a semiconductor substrate as a switching element, and FIG. 13B shows an equivalent circuit thereof. Referring to FIG. 13A, the Schottky barrier diode 21 serving as a switching element is disposed on one of the widthwise sides of a transmission line 5. The Schottky barrier diode 21 includes an anode electrode 11 extending perpendicular to the longitudinal direction of a transmission line 5, and a cathode electrode 13. The cathode electrode 13 is grounded through a via hole 17 connected thereto.
As shown in FIG. 13B, the terminal P is an RF signal input terminal, the terminal Q is an RF signal input terminal, and a node R is a junction of the Schottky barrier diode 21 and the transmission line 5. The Schottky barrier diode 21 is connected with a parasitic inductance L, and the terminal Q is connected with a bias circuit 41 for controlling the switching operation of the Schottky barrier diode 21. The bias circuit 41 includes a line 40 having a length equal to one-fourth of the wavelength of the RF input signal. The bias circuit 41 may be connected to the terminal P or the node R.
In a turn-off operation of the Schottky barrier diode 21 serving as a switching element, a positive voltage Vc (Vc greater than "PHgr"b ("PHgr"b: a Schottky barrier voltage)) is applied to a node T of the bias circuit 41. In this time, as shown in FIG. 14A, the Schottky barrier diode 21 may be deemed as an on-resistance Ron. On one hand, a negative voltage Vc (Vc less than 0) is applied to the node T of the bias circuit 41 to turn off the Schottky barrier diode 21. In this time, the Schottky barrier diode 21 may be deemed as an off-capacitance Colt, as shown in FIG. 14B.
The parasitic inductance component L primarily depends on the distance m between the transmission line 5 and the via hole (ground) 17 through the Schottky barrier diode 21, and affects particularly the isolation characteristic in the off-state. Based on FIG. 14A, an isolation value S21 in the off-state is represented as follows;
S21=2/(2+Z0/(Ron+jxcfx89L))xe2x80x83xe2x80x83(1)
xcfx89=2xcfx80f,xe2x80x83xe2x80x83(2)
where Z0 is a characteristic impedance, L is a parasitic inductance, and f is a frequency.
When Ron less than  less than Z0, the isolation value |S21| less than  less than 1 can be obtained in the low range of the frequency f. However, in the EHF band (fxe2x89xa730 GHz), there is a problem in that the isolation value S21 is reduced. FIG. 15 shows characteristics of an insertion loss in the on-state and an isolation value in the off-state. It is noted that in this drawing X and Y indicate the transmission loss and the isolation value, respectively.
In view of the above problem, it is therefore an object of the present invention to provide a semiconductor device including a switching element used in the EHF band which has an excellent isolation characteristic.
In order to achieve the object, in a semiconductor device of the present invention, a switching element includes an anode electrode, a cathode electrode and a via hole, wherein the anode and cathode electrodes are arranged to a transmission line so as to reduce the distance between the anode electrode and the via hole. More specifically, the anode electrode is arranged across the transmission line, within the transmission line, or in a region lying in an extending direction of the transmission line.
According to the first aspect of the present invention, provided is a semiconductor device which is mounted with a switching element operable to control transmission and cutoff of a signal transmitted between two portions of a transmission line. An anode electrode of the switching element is interposed between the two portions of the transmission line with a longitudinal direction of the anode electrode according with a longitudinal direction of the transmission line. A cathode electrode of the switching element is disposed on at least one of the widthwise sides of the anode electrode, and is connected to a ground. This arrangement allows the distance between the anode electrode and a via hole to be reduced, and thus effect of a parasitic inductance can be reduced and isolation characteristic in an off-state of the switching element can be improved.
According to the second aspect of the present invention, provided is semiconductor device which is provided with a switching element operable to control transmission and cutoff of a signal transmitted between two portions of a transmission line. An anode electrode of the switching element is interposed between the two portions of the transmission line with a longitudinal direction of the anode electrode perpendicular to a longitudinal direction of the transmission line. A cathode electrode of the switching element is disposed on at least one of the widthwise sides of the anode electrode, and is connected to a ground. This arrangement provides the same effect as that in the first aspect. In addition, the anode electrode in the second aspect is arranged to the transmission line differently by 90 degrees as compared to the anode-electrode arrangement in the first aspect. Thus, the transmission line can be arranged to extend in two different directions crossing at a right angle by using the first and second semiconductor devices in combination with each other, which provides increased flexibility of signal input/output directions.
In the above semiconductor devices, the cathode electrode may be connected to a via hole which can provide a ground connection.
In the above semiconductor devices, the transmission line may be a coplanar line including a signal line and a grounding line. In this case, the anode electrode is connected to the signal line, and the cathode electrode is connected to the grounding line. The coplanar line used as the transmission line can eliminate the need for the via hole, and thus can achieve simplified process and reduced cost in manufacturing.
In the above semiconductor devices, the switching element may have a plurality of anode electrodes. This can reduce on-resistance or enhance isolation characteristic of the Schottky barrier diode.
In the above semiconductor devices, a via hole may be formed within the cathode electrode. This can further reduce a distance between the anode electrode and the via hole and enhance isolation characteristic in the off-state.
In the first aspect, the two portions of the transmission line may be coupled each other through an air bridge over the switching element. This can enhance electrical connection between the anode electrode and the transmission line.
In the second aspect, the anode electrode may be coupled to the transmission line through an air bridge. This allows the anode electrode and the transmission line to be electrically connected with one another.
Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description.